Peelable laminated structures are used, in particular, in container and packaging technologies.
The use of containers made of metal, metal foil, plastic or glass and provided with peelable closure elements, such as covers, seals, lids or other sealing elements made of metal foil or flexible plastics, is widespread nowadays. For example, containers of this kind are used for packaging human and animal foodstuffs and for protecting articles which have to be kept sanitary or sterile, e.g. medical instruments or equipment, pharmaceutical doses, and the like. The advantage of using containers of this type is that the they are generally inexpensive, can be sealed hermetically and can be opened easily by hand or simple machine.
Sealed containers of this kind may be produced in a variety of types, shapes and sizes. For example, the containers may consist of rigid or semi-rigid shaped hollow bodies having thin flexible metal or plastic lids or may be in the form of flexible metal or plastic pouches having seams which can be peeled apart. The use of metal foils to form part or all of such containers is common because metal foils have good oxygen and moisture barrier properties, good mechanical and thermal properties and can be made to look attractive.
Peelable sealed packages are conventionally made by attaching a closure element to a container by means of an adhesive, a heat sealable lacquer or a heat sealable polymer membrane (usually a polypropylene membrane) having a sufficient adhesive strength to prevent the container from opening prematurely, but nevertheless having a strength low enough to permit the closure element to be peeled away from the container by hand or simple machine when the container is to be opened. Incidentally, to avoid unnecessary repetition, adhesives, lacquers and polymer membranes are referred to collectively throughout the following description and claims merely as "adhesives."
While many peelable adhesives are already known, not all are suitable for use in all cases. For example, the choice of a suitable adhesive is particularly difficult when the container is to be heated prior to being opened, for example if it contains a foodstuff to be served hot or if it contains an item to be pasteurized (heated for a short period at about 80.degree.-85.degree. C.) or sterilized (heated for a longer time usually at a temperature in the range of 120.degree.-130.degree. C.) in heated water or other heating medium. In such circumstances, the adhesive strength is often changed (usually substantially reduced, often by a factor of two or more) during the heating step, leading to a premature failure of the seal or to difficulty when opening the container. To make matters worse, the extent of the change in the adhesive strength is often difficult to predict as it can be very sensitive to variations in adhesive formulations and to variations of time and temperature of the heating process. The presence of moisture or steam can also have a significant effect on the adhesive strength.
Even when the container is not to be subjected to a heating step prior to use, the adhesive still has to be carefully chosen in order to provide a peel strength sufficient to provide an effective and durable seal, while at the same time allowing the container to be opened easily. In the case of products having a long shelf life, the choice of suitable adhesives may be further limited by the need to avoid adhesives which change adhesive properties with age, and in the case of containers intended to hold foodstuffs or other comestibles, the choice is limited to adhesives approved by health care regulations.
In consequence, it will be appreciated that a certain amount of care is required in the choice and application of adhesives for peelable containers, and this often translates into more expensive products or to a limitation of the use of peelable sealed containers for certain items or applications.
It would therefore be advantageous to provide peelable sealed containers incorporating peelable structures having peel characteristics which are less dependent on the choice of the sealing adhesives and which are less sensitive to variations in temperature and processing times.
Furthermore, there is a need for peelable structures of this kind which may be used for applications other than in peelable containers. For example, flexible packaging films often consist of one or more polymer layers laminated to a metal foil so that a desired combination of properties of both materials can be obtained. However, laminated materials of this kind are difficult or expensive to recycle and recyclability is becoming increasingly important both at the factory level (to deal with internally generated scrap, trimmings, etc.) and at the consumer level after sale and use. The difficulty is caused by the bonding together of two different types of materials (metal and plastics) which are recycled by different methods. It would therefore be advantageous to produce laminated films of this kind which could be easily separated into their component materials when desired simply by peeling the materials apart by hand in order to facilitate recycling.